Calcium transport across the cortical thick ascending limb of Henle's loop has been studied in isolated perfused segments of rabbit renal tubules in vitro. Radioisotopic flux studies demonstrate passive calcium transport driven by the transepithelial voltage. Parathyroid hormone increases calcium absorption by stimulation of adenylate cyclase. Although parathyroid hormone rectifies the passive, voltage-dependent calcium transport in the absorptive direction under the conditions of our experiments, the mechanism by which the rectification occurs is uncertain.